Electrical power distribution switchyards (or power substations) facilitate the distribution of power from a generation source (such as a hydroelectric dam or a coal plant) to utility customers. For example, in a hydroelectric power distribution system, power generated by the dam is initially routed to a primary power substation located near the dam. The primary power substation then distributes the power via power lines to secondary power substations located in various counties. The secondary power substations then further distribute the power to local power substations located in particular cities or towns. Distribution continues until the power is distributed to each residence in the cities and towns.
Power substations contain high voltage and high amperage power distribution equipment. Due to the presence of high voltage and amperage loads, the power distribution equipment poses a significant threat to the lives of utility personnel. Therefore, certain precautions must be made in the design and construction of substations to diminish the threat to personnel. Toward this end, utility companies issue specifications for the construction of power substations which ensure maximum safety of utility personnel.
One set of specifications concerns the surfacing of power substations. Substation surfacing consists of crushed aggregate (typically basalt or granite mineralogy), which is laid around the power distribution equipment, sidewalks, and building structures in the substation. The aggregate surfacing serves two purposes. First, and most importantly, the aggregate serves as an electrical barrier which protects utility companies' personnel in the event of an electrical fault. Second, the aggregate provides a suitable support surface for vehicles and portable utility equipment.
According to standard utility specifications, the aggregate surfacing must have a minimum resistivity of 3000 ohm-meters, which is considered to be sufficient to protect personnel in the event of an electrical fault. The resistivity of the aggregate surfacing is directly related to the size and corresponding voids ratio of the individual nuggets which make up the aggregate. Larger grade nuggets have higher resistivity than smaller grade nuggets. Therefore, aggregate surfacing having the larger grade nuggets, referred to as "coarse aggregate", has a higher resistivity than aggregate surfacing having smaller grade nuggets (or finer materials such as sand, silt, clay, volcanic ash, or organic material), referred to as "fine aggregate".
The minimum resistivity requirement is easily satisfied when the aggregate surfacing is first laid because only coarse aggregate is employed. Unfortunately, over time, fine aggregate infiltrates the coarse aggregate, thereby lowering the resistivity of the surfacing. The fine aggregate infiltrates the coarse aggregate in many ways. For example, the fine aggregate may be blown by the wind and deposited on the coarse aggregate surfacing. Alternatively, the weight of the coarse aggregate and the weight of utility personnel and vehicles may force the coarse aggregate down into a subgrade which consists primarily of fine aggregate.
As the fine aggregate accumulates in the coarse aggregate, the resistivity of the substation surfacing decreases below the minimum resistivity level (e.g., 3000 ohm-meters), thereby rendering the surfacing unsuitable. Therefore, the mixed fine and coarse aggregate surfacing must be removed and replaced with a new coarse aggregate surfacing which satisfies the minimum resistivity requirements.
Two techniques have been employed to replace unsuitable substation surfaces. One technique is simply to add more coarse aggregate to the top of the existing, unsuitable surface. This technique has a disadvantage in that extra coarse aggregate may be added only a few times since the increasing elevation of the aggregate surfacing would eventually rise above sidewalks, power equipment bases, substation fences, and building foundations.
A second technique is to remove and dispose of the unsuitable surfacing, and then lay an entirely new aggregate surfacing which satisfies the utility companies' requirements. The second technique is expensive, however, due to the high costs of disposing of the unsuitable surfacing which may be contaminated with toxicogenics.
Another disadvantage of both techniques is that adding a new aggregate surfacing is very expensive because the sources of coarse aggregate are limited and the transportation costs associated with transporting the coarse aggregate from the source to the remote power substation are high.
The present invention relates to a process for in situ recycling of aggregate surfacing used in power substations. According to the process of the present invention, the unsuitable surfacing is removed, recycled and then relaid as a suitable aggregate surfacing. In this manner, the high cost of purchasing and transporting new aggregate surfacing is eliminated. Furthermore, the process is very efficient because the entire resurfacing may be performed on site at the power substation.